1. Technical Field
The present invention pertains to beam control within electron microscopes. In particular, the present invention pertains to determining optimal beam parameters for a scanning electron microscope and automatically applying the optimal beam parameters to adjust the microscope beam for enhanced operation.
2. Discussion of Related Art
A scanning electron microscope (SEM) may be utilized to measure a dimension of a pattern in a sample, such as a semi-conductor device. Generally, the microscope directs electrons in the form of a beam to converge at a point on a microscope stage containing the sample or semi-conductor device. The electron beam is scanned across the stage, where electronic control mechanisms are typically housed within a column of the microscope and utilized to steer and focus the beam onto the sample. These mechanisms include various lenses and a cylindrical coil with additional exterior coils to produce electric fields to steer the beam along one or more axes toward the sample. The parameters of these mechanisms are critical to proper microscope operation for an application.
When the sample on the stage is irradiated, a physio-electrical reaction occurs and electrons are reflected and/or discharged from the sample surface. The microscope collects and detects the electrons from the sample surface to form an electronic intensity profile of that surface. Since various portions of the semi-conductor device produce different electron spectra (e.g., different material layers, different topography (e.g., flat areas, edges, etc.), etc.), the profile basically provides a view of the sample surface.
The scanning of the beam across a desired sample portion and subsequent collection of the reflected and/or discharged electrons enables measurement of the sample portion size. This feature is commonly known as a critical dimension (CD) and may be of any feature or characteristic of the sample. Electron microscopes performing these types of measurements are generally referred to as critical dimension scanning electron microscopes (CDSEM). Since uniformity and accuracy with respect to critical dimension measurement is crucial for device yield in semi-conductor processing, the scanning electron microscope is employed to provide an accurate critical dimension measurement. However, the quality of the beam image may degrade due to various causes (e.g., astigmatism of the beam system, reduced resolution attributed to defocusing, etc.), thereby rendering measurements unsatisfactory. Thus, set-up and maintenance of the electronic and physical hardware of the microscope for beam delivery and subsequent electron collection are crucial. Further, chamber conditions and substrate materials are considered since these have a significant effect on the optimization of the beam delivery parameters for each substrate layer.
Accordingly, frequent changes are required for the set points and alignment of the microscope beam. This process is generally performed manually. Specifically, a vendor typically provides an artifact on the microscope stage. The microscope scans a particular feature of the artifact (e.g., prior to processing of the desired semi-conductor device) and a technician subsequently adjusts set points of beam focusing and stigmation parameters (e.g., via adjustment of potentiometers). This is typically accomplished by altering a control current of an objective lens and control current of the above-described coils while observing the beam image. The technician is basically aligning the beam with respect to a plurality of axes to optimize the visual acuity of the sample on the stage. The visual acuity generally refers to the condition providing an optimal beam conditioning (e.g., the best beam focus, the least effect of stigmation, etc.) for the particular sample.
The related art suffers from several disadvantages. In particular, the set points and parameters are determined based on a subjective viewing of image quality by the technician. This tends to provide varying criteria for image quality and produce variations in the determined set points and parameters, thereby producing varying results for feature measurements. Further, microscope control settings determined from poor microscope images may result in unpredictable actions. Moreover, the microscope is typically monitored and/or calibrated periodically. Thus, the microscope may be utilized with marginal or inadequate settings for a particular sample and provide inaccurate measurement results. In addition, the manual process typically requires several iterations to attain desired settings and may be repeated several times in a short time interval, thereby significantly increasing the time for tasks and semi-conductor processing.